1. Field of the Invention
This invention relates to a pressure control device for a cryogenic liquid vessel and, more specifically, a pressure control device for a cryogenic liquid vessel wherein the economizer circuit utilizes a flow integration device to combine the gas from the vessel vapor space with liquid from the vessel liquid space.
2. Background Information
Cryogenic liquids, such as liquid natural gas (LNG), nitrogen, oxygen, CO2, hydrogen and the like, are substances that normally exist as gasses, but are liquids at cold temperatures. Special vessels and systems must be used to store and transfer cryogenic liquids because of difficulty in maintaining the extremely cold temperatures. Such vessels typically include a double walled vessel having a vacuum in the annular space. While the vacuum provides an effective insulation, the insulation is not perfect and, as such heat penetrates the vessel. When heat is added to the cryogenic liquid, a portion of the liquid returns to the gaseous state. The gas within the vessel increases the internal pressure. Eventually, to prevent over pressurization of the vessel, the gas must be vented. It is desirable to prevent, or at least delay, the venting of the gas.
Unless a cryogenic liquid vessel is merely a storage vessel, the cryogenic liquid vessel is typically coupled to a use device. For example, where the cryogenic liquid is LNG, the use device is typically an engine. The following description shall use the example of LNG and an engine, but it is understood that the system described herein is applicable to any cryogenic liquid and any use device. The fuel system for the engine includes the cryogenic vessel, a delivery line extending from the cryogenic vessel to the engine, a vaporizer on the delivery line and an economizer circuit. Within the cryogenic liquid vessel are a liquid space and a vapor space. The delivery line is in fluid communication with the cryogenic vessel liquid space and the economizer circuit is in fluid communication with both the cryogenic vessel vapor space and the delivery line. Because the engine uses the natural gas in a gaseous state, a vaporizer may be located on the delivery line extending from the cryogenic liquid vessel to the engine.
In operation, if the vessel does not have a sufficient pressure, a small quantity of cryogenic liquid may be removed from the liquid space, passed through a vaporizer where it converted to gas, and returned to the vapor space of the cryogenic liquid vessel. Alternatively, when the engine is not running, any excess cryogenic liquid from within the delivery line is allowed to evaporate and is returned to the vapor space through the economizer circuit. This gas pressurizes the cryogenic liquid vessel so that, when the engine is running, the pressure within the vessel causes the cryogenic liquid to exit the vessel to be delivered to the engine. Once the vessel is pressurized the delivery line may be opened to deliver LNG to the vaporizer or engine. Within the vessel, the vapor and the cryogenic liquid are at the same pressure. However, due to the additional pressure created by the weight of the cryogenic liquid, there is a slightly higher pressure acting on the delivery line. Thus, the path of least resistance to fluid flow is through the liquid portion of the delivery line and, when both the economizer circuit and the delivery line are open, fluid will flow from the liquid space within the vessel.
As noted above, heat causes the cryogenic liquid within the cryogenic liquid vessel to be converted to gas and may cause an undesired increase in pressure. To prevent the venting of gas to the atmosphere when the cryogenic vessel is over-pressurized, gas is removed from the vapor space within the cryogenic vessel and delivered to the engine. While a direct connection between the cryogenic vessel vapor space and the engine is possible, more typically, the gas is withdrawn through the economizer circuit. That is, the economizer circuit is in fluid communication with the delivery line. Thus, to deliver gas from the cryogenic liquid vessel vapor space, the flow of the cryogenic liquid through the delivery line is suspended to allow gas from the vapor space to travel through the economizer circuit into the delivery line and then to the engine. Typically, flow of the cryogenic liquid within the delivery line is stopped by a valve structured to sense the pressure within the cryogenic liquid vessel. When the pressure exceeds a set limit, the valve closes and flow of the cryogenic liquid is stopped. Pressure within the cryogenic vessel causes the gas within the vapor space to be expelled through the economizer circuit and delivered to the engine, thereby reducing the pressure within the cryogenic vessel. Thus, with this system the flow of the cryogenic liquid is stopped when vapor is withdrawn.
There is, therefore, a need for a cryogenic liquid vessel that does not stop the flow of the cryogenic liquid while removing gas from the cryogenic liquid vessel vapor space.
There is a further need for a cryogenic liquid vessel that combines the flow of the cryogenic liquid and the gas while removing gas from the cryogenic liquid vessel vapor space.
There is a further need for a cryogenic liquid vessel that includes a flow integration device to combine the flow of the cryogenic liquid and the gas while removing gas from the cryogenic liquid vessel vapor space.